Circuit overloads can overheat and thereby destroy semiconductor devices such as power transistors, voltage regulators, triacs and silicon controlled rectifiers (SCRs). Circuit designers are required to ensure that the semiconductor's maximum operating temperature, as specified by the manufacturer, is not exceeded, or risk destruction of the device.
Proper device selection will ensure that the electronic device will not overheat during normal circuit operation. In addition, mechanical devices can be used to prevent overheating, such as heat sinks and cooling fans. Typically, heat sinks are constructed of high thermal conductivity materials (most metals, for instance), and are attached directly to the electronic device to remove the generated heat away from the semiconductor. Cooling fans force the generated heat away from the device. However, for economic or spatial reasons, designs cannot always be incorporated to prevent overheating. Moreover, these mechanisms usually only attend to normal operating conditions.
Under abnormal conditions, especially severe cases such as when the load becomes short-circuited for instance, these protection mechanisms are unable to prevent the device from being destroyed through overheating. In addition, devices attached to a damaged semiconductor can themselves be damaged, electrically and/or mechanically, when a semiconductor burns up. The damage, and the expense in repairing this damage, can be prevented by sensing the temperature of the semiconductor when its temperature approaches or exceeds the specified maximum operating temperature. This invention describes the use of a particular temperature detector element to provide overtemperature protection for semiconductors.
Smaller detector elements are needed for these applications due to their shorter thermal response times, and because there is only a small amount of area available near some semiconductor packages for detector placement. The detector elements most likely to meet this requirement are NTC and PTC thermistors (negative and positive temperature control thermistors), resistive temperature devices (RTDs), and thermocouple junctions, which convert temperature into an electrical parameter (resistance or voltage, for instance). RTDs are very accurate, but are expensive because of the material used, platinum, for instance. Thermocouple junctions require elaborate electronics to perform the conversion from temperature to voltage because the voltage generated is at low levels (millivolts), requiring amplification and noise suppression. We have found that a thermistor is probably the best device available for this application because of its ready availability, reasonable cost and small size.
The difference between the two types of thermistors is that NTCs have a negative temperature coefficient (device resistance decreases as its temperature increases), and PTCs have a positive temperature coefficient of resistance. In addition, the magnitude of these coefficients can be very different for readily available devices, both of which are made of certain types of ceramic. The magnitude of the coefficient (i.e., the slope of the resistance vs. temperature curve) for a PTC is much greater than that of an NTC. This can be important in determining the complexity of the interface electronics between the detector and the semiconductor drive circuitry. The PTC, with its steeper slope, is more sensitive to temperature changes (i.e. its resistance change is larger for a given temperature change), so the necessary electronics would be simpler than that needed for an NTC.
Placement of the detector is very important to the overall success of the protection scheme. One reason is that temperature measurement accuracy increases as the distance between the detector and the semiconductor decreases, since the material between these two devices exhibits some "thermal resistance" that results in a temperature difference between the devices, even in the steady-state. Even more importantly, the response time of the detector to sudden changes in semiconductor temperature will decrease as the distance between the two devices decreases, since the heat transfer time between a semiconductor that becomes suddenly hot to a cooler detector directly depends on the separation distance.
There are other factors needing consideration when designing a housing for the detector itself. The thermal conductivity of the housing material must keep the overall thermal response time of the detector within acceptable limits (the thermal conductivity of the semiconductor package must be accounted for, as well). The detector/semiconductor/heat sink assembly must also be simple in order to keep production assembly costs low. Detector housing designs requiring few additional parts and assembly steps will be preferred.
A complication in selecting the housing material of the detector is that electrical isolation between the detector and the semiconductor is usually necessary. Most power semiconductor packages include a metal piece that not only conducts heat away from the semiconductor die to a heat sink, but is also electrically connected to one of the terminals of the device. Thus, if electrical isolation is needed between the detector and the semiconductor, the detector housing will have to be made of a very low electrically conductive material (plastic or epoxy, for instance) to provide the electrical isolation. These materials have a significantly lower thermal conductivity, however, and result in longer thermal response times. The trade-off between thermal response time and electrical isolation makes the eventual location of the detector even more important, especially for those applications needing electrical isolation and fast response times.
The overall detector package assembly cost can also be affected by these thermal and electrical requirements, since some applications might require a fast enough thermal response that only a metal (electrically conductive) can provide. We have found that the detector itself must be thinly coated with epoxy before it can be inserted into the metal housing so electrical isolation is provided. This configuration will give fast heat transfer capabilities and still provide electrical isolation, but at additional cost because of the extra precoating step needed before insertion into the housing.
In summary, detector selection will depend on the device's temperature sensitivity and measurement accuracy, the device's physical size, the necessary electronic interface circuitry and economic requirements (in both material and assembly costs) of the application, while the detector package design must account for the thermal, electrical and economic needs of the application. The detector and package described by the present invention can prevent the destruction of semiconductors by monitoring their temperature, and through associated electronic circuitry, shut them off when their maximum operating temperature is approached. The resulting package is easy to attach to standard semiconductor packages, and can be produced at reasonable cost.